Scientists have determined that these volcanoes occur along the boundaries of huge, moving slabs of crust, or plates, particularly where an ocean plate is plunging beneath a continental plate. This process is called subduction. Heat generated by this process yields magma (molten rock) that rises to the surface. In addition, sudden movements between the plates cause strong earthquakes in many of the same areas that experience volcanic eruptions.

Volcanoes may also form where ocean plates are spreading apart. Many of these eruptions take place on the ocean floor and are not seen by man. However, if you live in the island country of Iceland, you are perched atop the Reykjanes Ridge, which connects to the Mid-Atlantic Ridge, where the plates that include North and South America are moving apart from the ones that include Europe and Africa. In a few other instances, isolated hot spots beneath crustal plates have generated large volcanoes in Hawaii and on the African continent.

Generally, more dangerous explosive eruptions are produced by magma rich in silica. This kind of magma has a stiff consistency, and it can temporarily clog a volcano until the gases build up sufficient pressure to blast open the volcano. Silica-rich magma solidifies into light-colored rocks and is common for volcanoes along plate margins. Explosions can also result when rising magma encounters water and flashes it to steam. Hot ash produced by explosive eruptions can be deadly—three volcanoes in the Caribbean-Central America region killed more than 36,000 people in a six-month period in 1902.

On the other hand, oceanic hot-spot and plate-spreading volcanoes, and many others, are largely made up of dark basalt, which is low in silica but rich in iron and magnesium. Basalt magma is fluid and generally results in mildly explosive or nonexplosive eruptions and also slow-moving lava flows that are relatively easy for people to avoid. Yet, these eruptions may be long-lived—Kilauea volcano on the island of Hawaii has been erupting continually since January 1983. Although extensive property damage has resulted from such eruptions, they rarely result in injury or death.